Breakdown:
1: Om, that (Divine Illumination) which Pervades the Bhu Loka (Physical Plane), Bhuvar Loka (Antariksha Loka or the Astral Plane) and Suvar Loka (Swarga Loka or the Celestial Plane),
2: That Savitr (Divine Illumination) which is the Most Adorable,
3: On that Divine Radiance we Meditate,
4: May that Enlighten Our Intellect and Awaken our Spiritual Wisdom

Meaning:
We meditate on the transcendental Glory of the Deity Supreme, who is inside the heart of the earth, inside the life of the sky and inside the soul of Heaven. May He stimulate and illuminate our minds.

OR:

OM. I adore the Divine Self who illuminates the three worlds — physical, astral and causal; I offer my prayers to that God who shines like the Sun. May He enlighten our intellect.

According to the Vedas, there are seven realms or spheres or planes of existence, each more spiritually advanced than the previous one. It is written that through spiritual awareness and development, we can progressively move through these realms and ultimately merge with the Supreme Being. Many Buddhist teachings have also referred to these seven realms.

The Gayatri mantra is one of the oldest and most powerful of Sanskrit mantras. It is believed that by chanting the Gayatri mantra and firmly establishing it in the mind, if you carry on your life and do the work that is ordained for you, your life will be full of happiness.

The word “Gayatri” itself explains the reason for the existence of this mantra. It has its origin in the Sanskrit phrase Gayantam Triyate iti, and refers to that mantra which rescues the chanter from all adverse situations that may lead to mortality.

Goddess Gayatri is also called “Veda-Mata” or the Mother of the Vedas – Rig, Yajur, Saam and Atharva – because it is the very basis of the Vedas. It is the basis, the reality behind the experienced and the cognized universe.

The Gayatri mantra is composed of a metre consisting of 24 syllables – generally arranged in a triplet of eight syllables each. Therefore, this particular meter (tripadhi) is also known as the Gayatri Meter or “Gayatri Chhanda.”

By chanting this mantra, it is said that Divine spiritual light and power is infused in each of our seven chakras and connects them to these seven great spiritual realms of existence.

It is true that Gayatri Mantra is in Rigveda not in one placee but many places: Many people ascribe it to Vishwamitra Rishi : In south india Sri Gayatridevi is depicted as a Goddess (Female) with FIVE Faces :The Mantra is not Gayathri Mantra as many people believe,It is Chandas(Gayathri Chandas) The Rishi is Vishwamitra, The God is Savitar ie Surya Bhagawan : Even Sri Rama worshiped Surya Bhagawan(AS advised by sage Aghastya Rishi) Aditya Hridaya is a very important Stotra : Valmiki in Ramayana, beautifully Explains the Aditya Hridaya : Sri Rama over powered Ravana & Killed him after reciting Aditya Hridaya///

This brief write up on Gayatri Mantra does not mention the great Rishi Vishwamitra who composed the glorious mantra. He is reported to have included all the fourteen material lokas in it at first and later reduced to three- bhu, bhuvah and suvah only. Please give utmost importance to the mantra drishta Vishwamitra Maharishi, a Kshatriya king, who rose to the pinnacle of a Brahmarishi and the one who composed Gayatri first and later this found reference in the Veda. Just because we have lost the power to see the Rishis, it does not mean that they don’t exist at present. Om Namo Namah…